Sierra snow conditions create rare avalanche problem

Expert: Type of widespread weak layer only seen three times in 12 years has happened four times this season

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Owner of the Tahoe Mountain School and official observer for the Sierra Avalanche Center Steve Reynaud checks the exact degree of a northeast slope on Mount Tamarack near the top of Mt. Rose Highway south of Reno on Feb. 24, 2015.(Photo: Jason Bean/RGJ)Buy Photo

But this month, in certain locations across the forecast region, the frost managed to survive subsequent snowstorms. That means it’s buried now and forming weak layers that can break and create slab avalanches.

Schwartz said in the past 12 seasons widespread buried surface hoar conditions have only occurred three times. This season it’s already happened four times. In the Sierra Nevada it's much more common to encounter avalanche problems related to storm and wind loaded snow.

“(Surface hoar) is generally destroyed by wind and warm temperatures leading up to the next storm,” he said. “(Recent storms) have come in with less wind and colder temperatures.”

Schwartz said the reason it’s important to be aware of the condition is because it’s so unusual for the Sierra Nevada.

Backcountry clearings below treelines, which are often the types of places skiers, snowboarders and snowmobilers love to ride, are where the persistent slabs are forming.

“The backcountry public needs to approach differently and manage terrain differently when dealing with this type of weak layer,” Schwartz said.

With more storms expected to roll into the region beginning Sunday conditions could become more treacherous, at least for awhile.

If the storms dump enough snow to trigger widespread avalanches throughout the region it could have a “cleanup” effect on the persistent slab problem, which could lead to the snowpack stabilizing once the storm-related conditions are past.

Another potential scenario, however, is that the storms bring enough snow to load more depth onto the existing unstable slabs but not enough to trigger them.

That would mean the persistent layer would remain present and at risk for human triggers.

“It can remain weak and reactive for weeks,” Schwartz said.

More information on snowpack observations, advisories, forecasts and safe backcountry winter travel is available at www.sierraavalanchecenter.org